1. Field of the Invention
The present invention relates to a method for preparing nonionic water-soluble cellulose ether having low insoluble fiber content and a high utilization ratio of an etherifying agent.
2. Description of the Related Art
When nonionic water-soluble cellulose ether is dissolved in water, it becomes tacky so that it is used for a thickener of transparent shampoos or rinses, hair dressing agents, eye drops, detergents for contact lens and the like. For example, nonionic water-soluble cellulose ether such as methyl cellulose or hydroxypropyl cellulose comprises a hydrophilic group and a hydrophobic group in the molecule thereof, thereby showing interfacial activity. It is therefore used as a suspension stabilizer in suspension polymerization of vinyl chloride or vinylidene chloride, and also used as a raw material for transparent wrap materials for domestic use. Products are desirably transparent in such applications. If an aqueous solution of nonionic water-soluble cellulose ether does not show molecular level dissolution and is therefore not transparent, products obtained may be inferior in transparency or in function due to occurrence of defective portions in the products. Although an aqueous solution of nonionic water-soluble cellulose ether is required to have a high viscosity, it is considered to be difficult to obtain a transparent aqueous solution because high-viscosity nonionic water-soluble cellulose ether has higher insoluble fiber content than low-viscosity cellulose ether.
With a view to overcoming these problems, according to Japanese Patent Application Examined Publication (JP-B) No. 53-12954/1978, proposed is a method comprising steps of allowing a raw material pulp to adsorb an aqueous alkali solution having a concentration of 15 to 75% by weight at a temperature of 5 to 80° C.; pressing the resulting pulp within 10 seconds after adsorption to remove an excess portion of the aqueous alkali solution; repeating the above steps to obtain alkali cellulose, and reacting the alkali cellulose with an etherifying agent.
According to Japanese Patent Application Unexamined Publication (JP-A) No. 10-259201/1998, proposed is a method comprising steps of impregnating a pulp having dichloromethane extract content of 0.07% by weight or less in an aqueous sodium hydroxide solution, pressing the resulting pulp to obtain alkali cellulose, and then etherifying the alkali cellulose.
According to Japanese Patent Application Unexamined Publication (JP-A) No. 2001-354701, nonionic water-soluble ether is obtained by grinding a pulp sheet having a sheet density of 0.4 to 1.0 g/ml into powders having an average particle size of 1000 μm or less, adding an alkali to the powders to obtain the alkali cellulose, and then reacting the alkali cellulose with methyl chloride, propylene oxide and the like.
A. W. Anderson and R. W. Swinehart, Tappi, Vol. 39, No. 8, 548-553, August, 1956, describes a method of impregnating a pulp sheet having a sheet density of 0.47 to 1.17 g/ml in a bath containing an alkali solution for 0.5 to 4.5 seconds to obtain alkali cellulose.
According to U.S. Pat. No. 2,102,205, pulp impregnated in an aqueous sodium hydroxide solution for 2 hours is pressed to obtain alkali cellulose.